This invention relates to the production of alkyl mercaptocarboxylate/dialkyl thiodicarboxylate mixtures, or alkyl mercaptocarboxylate/dialkyl thiodicarboxylate/dialkyl dithiodicarboxylate mixtures. The production of mercapto derivatives of unsaturated esters as a transitory intermediate in the production of thiodiesters is broadly known as disclosed in U.S. Pat. No. 2,845,390. It has been found that polymers suitable for sealants can be prepared by transesterification using a mixture of alkyl mercaptocarboxylates and dialkyl thiocarboxylates with a poly(oxyalkylene)-polyol. However, the resulting product, on curing, is too brittle if the mixture contains too much of the dialkyl thiodicarboxylate.